[R] How to get row index of sorted matrix

[jim holtman]

Is this what you were looking for:

> x <- scan(textConnection(" 1372.23718 2.277450e+04
+    74.48333 5.494448e+01
+   226.63590 6.023199e+02
+  1947.17564 4.044391e+04
+   178.79615 3.970006e+02
+   657.56857 3.396852e+04
+   251.60519 1.239538e+03
+    78.53846 7.473607e+01
+   140.37564 2.395830e+02
+    358.13718 1.508718e+03"), what=0)
Read 20 items
> closeAllConnections()
> mydata <- matrix(x, ncol=2, byrow=TRUE)
> mydata
            [,1]        [,2]
 [1,] 1372.23718 22774.50000
 [2,]   74.48333    54.94448
 [3,]  226.63590   602.31990
 [4,] 1947.17564 40443.91000
 [5,]  178.79615   397.00060
 [6,]  657.56857 33968.52000
 [7,]  251.60519  1239.53800
 [8,]   78.53846    74.73607
 [9,]  140.37564   239.58300
[10,]  358.13718  1508.71800
> # top 3 indices
> order(mydata[,2], decreasing=TRUE)[1:3]
[1] 4 6 1
> # print out the ordered data
> mydata[order(mydata[,2], decreasing=TRUE),]
            [,1]        [,2]
 [1,] 1947.17564 40443.91000
 [2,]  657.56857 33968.52000
 [3,] 1372.23718 22774.50000
 [4,]  358.13718  1508.71800
 [5,]  251.60519  1239.53800
 [6,]  226.63590   602.31990
 [7,]  178.79615   397.00060
 [8,]  140.37564   239.58300
 [9,]   78.53846    74.73607
[10,]   74.48333    54.94448
>


On Mon, Jun 23, 2008 at 9:35 PM, Gundala Viswanath <gundalav at gmail.com> wrote:
> Hi,
>
> How can I get the top-3 index (sorted by "var") of the following
> matrix?
>
>> print(mydata)
>                                 var
>  [1,]  1372.23718 2.277450e+04
>  [2,]    74.48333 5.494448e+01
>  [3,]   226.63590 6.023199e+02
>  [4,]  1947.17564 4.044391e+04
>  [5,]   178.79615 3.970006e+02
>  [6,]   657.56857 3.396852e+04
>  [7,]   251.60519 1.239538e+03
>  [8,]    78.53846 7.473607e+01
>  [9,]   140.37564 2.395830e+02
>  [10,]   358.13718 1.508718e+03
>
>
> - Gundala Viswanath
> Jakarta - Indonesia
>
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>



-- 
Jim Holtman
Cincinnati, OH
+1 513 646 9390

What is the problem you are trying to solve?